In the abovementioned systems, the users of one and the same service S share a common key providing access to this service. This traffic key must be renewed periodically in order to ensure the security of the system.
A management center is capable of broadcasting a new traffic key value buried in the service S itself, by using a limited bandwidth.
The key management for the traffic keys requires the resolution of the following compromise:                periodically, session after session, the key must be changed and access to the service must be controlled,        access to the service must be made as transparent as possible for authorized users who may not always be connected to the service.        
The main object of this function called OTAR (“Over The Air Rekeying”) is to provide the authorized users with transparent access to the service. For a given user, access is all the more transparent:                if he is capable of retaining the knowledge of the future traffic keys by preventing the use of other means of distribution than the service S itself, such as a manual offline distribution of the keys for example;        if he can sometimes be disconnected and consequently lose several messages broadcast by the OTAR function.        